Continuous electrolytically regenerated packed bed suppressor for ion chromatography

ABSTRACT

An electrolyitic suppressor including (a) a suppressor bed of ion exchange resin, (b) an electrode chamber adjacent the suppressor, (c) a first electrode in the electrode chamber, (d) a barrier separating the suppressor bed from the first electrode chamber preventing significant liquid flow but permitting transport of ions only of the same charge as the suppressor bed resin, (e) a second electrode in electrical communication with the resin bed, and (f) a recycle conduit between the suppressor outlet port and said electrode chamber. The second electrode may be in contact with the ion exchange resin in the suppressor or located in second electrode chamber. For anion analysis, the method of using the apparatus includes: (a) flowing an aqueous liquid sample stream containing anions to be detected and cation hydroxide through the separator bed, (b) flowing the aqueous effluent from the separator bed through the flow-through suppressor, (c) flowing the effluent liquid from the suppressor past a detector, (d) recycling said liquid effluent from the detector through a cathode chamber proximate to the suppressor bed and separated by the first barrier, and (e) applying an electrical potential between the cathode and the anode. Water is electrolyzed at the anode to cause cations on the cation exchange resin to electromigrate toward the barrier and to be transported across the barrier toward the cathode while water in the cathode chamber is electrolyzed to generate hydroxide ions which combine with the transported cations to form cation hydroxide in the cathode chamber.

BACKGROUND OF THE INVENTION

The present invention relates to method and apparatus using continuoussuppression of electrolyte in eluents particularly for the analysis ofanions or cations in ion chromatography.

Ion chromatography is a known technique for the analysis of ions whichtypically includes a chromatographic separation stage using an eluentcontaining an electrolyte, and an eluent suppression stage, followed bydetection, typically by an electrical conductivity detector. In thechromatographic separation stage, ions of an injected sample are elutedthrough a separation column using an electrolyte as the eluent. In thesuppression stage, electrical conductivity of the electrolyte issuppressed but not that of the separated ions so that the latter may bedetermined by a conductivity cell. This technique is described in detailin U.S. Pat. Nos. 3,897,213, 3,920,397, 3,925,019 and 3,926,559.

Suppression or stripping of the electrolyte is described in the aboveprior art references by a bed of ion exchange resin particles commonlyreferred to as a packed bed suppressor (PBS). The PBS requires periodicregeneration by flushing with an acid or base solution.

While packed bed suppressors have proven useful in ion chromatography,there are a number of disadvantages of a PBS. These disadvantagesinclude a) periodic regeneration of the PBS which interrupts sampleanalysis, b) a loss of resolution due to band broadening in the PBS andc) changes in retention of certain analytes as a function of the degreeof exhaustion of the PBS.

The volume and capacity of the PBS is generally large relative theseparation column to contain sufficient ion exchange resin so that thesuppression reaction can be performed for a large number of analysis(e.g. 15 to 50) prior to regeneration. By making the volume and capacityof the suppressor sufficiently large, the need to regenerate is lessfrequent which permits a larger number of samples to be analyzed beforethe system must be disrupted to regenerate the suppressor. Regenerationtypically requires placing the suppressor out of line of the analyticalsystem and pumping a concentrated acid or base solution (regenerant)through the suppressor.

If the suppressor's void volume is too large, the separation of theanalytes achieved in the separator column is compromised due tore-mixing of the analytes in the void volume, resulting in lowerresolution. Thus, the suppressor volume is a compromise betweenregeneration frequency and chromatographic resolution.

The regeneration process typically requires 20-60 minutes, depending onthe volume of the suppressor. A strong acid or base solution is firstpumped through the PBS in order to convert the resin to the acid (H₃O⁺)or base (OH⁻) form. After this conversion, deionized water is pumpedthrough the suppressor until any traces of the highly conductive acid orbase regenerant have been removed. The PBS is then placed back in linewith the analytical system and is allowed to equilibrate before sampleanalysis is performed.

In U.S. Pat. Nos. 5,597,734 and 5,567,307, a method is described ofregenerating a packed bed suppressor after each analysis. In thisapparatus, the packed bed suppressor has limited capacity for just oneor several sample analysis before the suppressor requires regeneration.The liquid flow through the low volume packed bed suppressor is usedwith suitable valving to pass liquid stream through the system. Duringanalysis, eluent from the separator passes through the suppressor and tothe conductivity cell. Immediately after the analysis, valving diverts aflow of chemical regenerant through the suppressor for regeneration. Thevalving then diverts eluent to the suppressor for equilibration prior tosample analysis. The regeneration and equilibration of this type of PBScan be performed in a short time with a small volume PBS.

Another form of packed bed suppression uses intermittent electrolyticregeneration as described and published in U.S. Pat. No. 5,633,171. Acommercial product using this form of suppression is described in“Electrochemically regenerated solid-phase suppressor for ionchromatography” Saari-Nordhaus, R. and Anderson, J. M., AmericanLaboratory, February 1996. In this product, an electrical potential isapplied through the resin in the packed bed suppressor while flowing anaqueous liquid stream to electrolyze water in the stream. For theanalysis of anions, a PBS containing fully sulfonated cation exchange isfitted with a cathode embedded in the resin at the suppressor inlet andan anode embedded in the resin at the suppressor outlet. Hydronium ionsgenerated at the anode displace the sodium ions which associate with thehydroxide ions for passage to waste, in this instance through theconductivity cell. This process electrochemically regenerates thesuppressor, and after the electrical potential is turned off, the devicecan be used as a conventional PBS. In a further embodiment, a second ionexchange resin bed is used with suitable valving to pass liquid streamsthrough the system. In one alternative of this system, a second samplein an eluent stream is chromatographically separated, typically on achromatographic column using an eluent. The eluent and separated secondsample flow through a second packed bed suppressor including ionexchange resin to convert the electrolyte to weakly ionized form. Then,the separated sample ionic species in the suppressor effluent aredetected in the detector. The effluent then flows through the firstpacked bed suppressor, forming the aqueous liquid stream required forregeneration and an electrical potential is applied and regeneration ofthe first packed bed suppressor is accomplished. The second suppressormay be similarly regenerated by positioning it after the detection celland flowing through the detector effluent of the first sample andapplying an electrical potential. This form of suppression does notrequire an external regenerant source and allows for uninterruptedoperation although it is not considered continuous. This system uses twoPBS's, additional valving and electronics to control the valve switchingand timing.

A different form of a suppressor is described and published in U.S. PatNo. 4,474,664, in which a charged ion exchange membrane in the form of afiber or sheet is used in place of the resin bed. The sample and eluentare passed on one side of the membrane with a flowing regenerant on theother side, the membrane partitioning the regenerant from the effluentof the chromatographic separation. The membrane passes ions of the samecharge as the exchangeable ions of the membrane to convert theelectrolyte of the eluent to weakly ionized form, followed by detectionof the ions.

Another suppression system is disclosed in U.S. Pat. No. 4,459,357.There, the effluent from a chromatographic column is passed through anopen flow channel defined by flat membranes on both sides of thechannel. On the opposite sides of both membranes are open channelsthrough which regenerant solution is passed. As with the fibersuppressor, the flat membranes pass ions of the same charge as theexchangeable ions of the membrane. An electric field is passed betweenelectrodes on opposite sides of the effluent channel to increase themobility of the ion exchange. One problem with this electrodialyticmembrane suppressor system is that high voltages (50-500 volts DC) areused. As the liquid stream becomes deionized, electrical resistanceincreases, resulting in substantial heat production. Such heat can bedetrimental to effective detection because it increases noise anddecreases sensitivity.

In U.S. Pat. No. 4,403,039, another form of electrodialytic suppressoris disclosed in which the ion exchange membranes are in the form ofconcentric tubes. One of the electrodes is at the center of theinnermost tube. One problem with this form of suppressor is limitedexchange capacity. Although the electrical field enhances ion mobility,the device is still dependent on diffusion of ions in the bulk solutionto the membrane.

Another form of suppressor is described in U.S. Pat. No. 4,999,098. Inthis apparatus, the suppressor includes at least one regenerantcompartment and one chromatographic effluent compartment separated by anion exchange membrane sheet. The sheet allows transmembrane passage ofions of the same charge as its exchangeable ions. Ion exchange screensare used in the regenerant and effluent compartments. Flow from theeffluent compartment is directed to a detector, such as an electricalconductivity detector, for detecting the resolved ionic species. Thescreens provide ion exchange sites and serve to provide site to sitetransfer paths across the effluent flow channel so that suppressioncapacity is no longer limited by diffusion of ions in the bulk solutionto the membrane. A sandwich suppressor is also disclosed including asecond membrane sheet opposite to the first membrane sheet and defininga second regenerant compartment. Spaced electrodes are disclosed incommunication with both regenerant chambers along the length of thesuppressor. By applying an electrical potential across the electrodes,there is an increase in the suppression capacity of the device. Thepatent discloses a typical regenerant solution (acid or base) flowing inthe regenerant flow channels and supplied from a regenerant deliverysource. In a typical anion analysis system, sodium hydroxide is theelectrolyte developing reagent and sulfuric acid is the regenerant Thepatent also discloses the possibility of using water to replace theregenerant solution in the electrodialytic mode.

Another improvement in suppression is described in U.S. Pat. No.5,248,426. This form of suppressor was introduced in 1992 by DionexCorporation under the name “Self Regenerating Suppressor” (SRS). Adirect current power controller generates an electric field across twoplatinum electrodes to electrolyze water in the regenerant channels.Functionalized ion-exchange screens are present in the regenerantchambers to facilitate electric current passage with permselectiveion-exchange membrane defining the chromatography eluent chamber, as inthe '098 patent. After detection, the chromatography effluent isrecycled through the suppressor to form a flowing sump for electrolyteion as well as providing the water for the electrolysis generating acidor base for suppression. Thus, no external regenerant is required andthe suppressor is continuously regenerated.

In copending application, Ser. No. 08/925,813, filed Sep. 4, 1997, nowabandoned entitled Ion Chromatographic Method and Apparatus Using aCombined Suppressor and Eluent Generator, incorporated herein byreference (“the copending application”), method and apparatus areprovided for generating an acid or base eluent in an aqueous solutionand for simultaneously suppressing conductivity of the eluent in an ionexchange bed after chromatographic separation in an ion chromatographysystem. Referring first to the apparatus, the suppressor and eluentgenerator comprises: a flow-through suppressor and eluent generator bedof ion exchange resin having exchangeable ions of one charge, positiveor negative, having an inlet and an outlet section in fluidcommunication with fluid inlet and outlet conduits, respectively; anelectrode chamber disposed adjacent to said suppressor and eluentgenerator bed inlet section and having fluid inlet and outlet ports; aflowing aqueous liquid source in fluid communication with said electrodechamber inlet port; a first electrode disposed in said electrodechamber; a barrier separating said suppressor and eluent generator bedfrom said electrode chamber, the barrier preventing significant liquidflow but permitting transport of ions only of the same charge as saidsuppressor and eluent generator bed resin exchangeable ions; and asecond electrode in electrical communication with said resin bed outletsection.

In one embodiment of the copending application ion chromatographyapparatus, the generator is used with a flow-through separator bed ofion exchange resin having exchangeable ions of opposite charge to theexchangeable ions of said suppressor and eluent generator bed, saidseparator bed having a sample inlet port and an effluent outlet port,said electrode chamber outlet port being in fluid communication withsaid separator bed inlet port, said separator bed outlet being in fluidcommunication with said suppressor and eluent generator bed inlet port,and a detector downstream from the generator. The aqueous liquid sourcecan be an independent reservoir or can be a recycle conduit from thedetector.

For anion analysis, one method includes (a) flowing an aqueous liquidsample stream containing anions to be detected and cation hydroxidethrough a separator bed of anion exchange resin with exchangeable anionsto form liquid effluent including separated anions and said cationhydroxide; (b) flowing said aqueous effluent from said separator bedthrough a flow-through suppressor and eluent generator bed comprisingcation exchange resin including exchangeable hydronium ions, so thatsaid cation hydroxide is converted to weakly ionized form, and some ofsaid exchangeable hydronium ions are displaced by cations from saidcation hydroxide, said suppressor and eluent generator bed having inletand outlet sections and inlet and outlet ports, liquid effluent fromsaid suppressor and eluent generator bed flowing through said outletport; (c) flowing an aqueous liquid through a cathode chamber proximateto said suppressor and eluent generator bed inlet section and separatedby a barrier therefrom, said barrier substantially preventing liquidflow between said cathode chamber and said suppressor and eluentgenerator bed inlet section while providing a cation transport bridgetherebetween; (d) applying an electrical potential between a cathode insaid cathode chamber and an anode in electrical communication with saidsuppressor and eluent generator bed outlet section, whereby water iselectrolyzed at said anode to generate hydronium ions to cause cationson said cation exchange resin to electromigrate toward said barrier andto be transported across said barrier toward said cathode in saidcathode chamber while water in said chamber is electrolyzed to generatehydroxide ions which combine with said transported cations to formcation hydroxide in said cathode chamber; (e) flowing said cationhydroxide from said cathode chamber to the inlet of said separatorcolumn; and (f) flowing the effluent liquid from said suppressor andeluent generator bed past a detector in which said separated anions aredetected.

After passing the detector in step (f), the effluent liquid can berecycled to said cathode chamber. The system can be used for cationanalysis by appropriate reversal of the cation and anion functionalcomponents.

In a second embodiment of the copending application suppressor andeluent generator bed, the second electrode is not in direct contact withthe suppressor and eluent generator bed. Instead, it is adjacent thesuppressor and eluent generator bed outlet section in a second electrodechamber similar to the one described above. In this embodiment, aqueousliquid exiting the detector may be recycled to the inlet of the secondelectrode chamber.

In a third embodiment, similar to the second one, aqueous liquid from areservoir is pumped to the inlet of the second electrode chamber. Liquidfrom the outlet of the second electrode chamber is directed to the inletof the first electrode chamber. Liquid flowing out of the firstelectrode chamber is directed to the inlet of the separator bed.

The copending application also discloses a method of anion analysisusing two electrode chambers separated from the suppressor and eluentgenerator bed which includes the following steps: (a) flowing an aqueousliquid sample stream containing anions to be detected and a cationhydroxide through a separator bed of anion exchange resin withexchangeable anions to form a liquid effluent including separated anionsand said cation hydroxide; (b) flowing said aqueous liquid effluent fromsaid separator bed through a flow-through suppressor and eluentgenerator bed comprising cation exchange resin including exchangeablehydronium ions, so that said cation hydroxide is converted to weaklyionized form, and some of said exchangeable hydronium ions are displacedby cations from said cation hydroxide, said suppressor and eluentgenerator bed having inlet and outlet sections and inlet and outletports, liquid effluent from said suppressor and eluent generator bedflowing through said outlet port; (c) flowing an aqueous liquid throughan anode chamber proximate to said suppressor and eluent generator bedoutlet section and separated by a first barrier therefrom, said firstbarrier substantially preventing liquid flow between said anode chamberand said suppressor and eluent generator bed outlet section whileproviding a cation transport bridge therebetween, said aqueous liquidexiting said anode chamber as an anode chamber aqueous liquid effluent;(d) flowing an aqueous liquid through a cathode chamber proximate tosaid suppressor and eluent generator bed inlet section and separated bya second barrier therefrom, said second barrier substantially preventingliquid flow between said cathode chamber and said suppressor and eluentgenerator bed inlet section while providing a cation transport bridgetherebetween; (e) applying an electrical potential between an anode insaid anode chamber and a cathode in said cathode chamber, whereby wateris electrolyzed at said anode to generate hydronium ions which aretransported across said first barrier to cause cations on said cationexchange resin to electromigrate toward said second barrier and to betransported across said second barrier toward said cathode in saidcathode chamber while water in said cathode chamber is electrolyzed togenerate hydroxide ions which combine with said transported cations toform cation hydroxide in said cathode chamber; (f) flowing said cationhydroxide from said cathode chamber to the inlet of said separator bed;and (g) flowing the effluent from said suppressor and eluent generatorbed past a detector in which said separated anions are detected.

The anode chamber aqueous liquid effluent may be recycled through saidcathode chamber. Alternatively, after detection in step (g), thesuppressor and eluent generator bed effluent may be recycled throughsaid anode chamber.

The history of ion chromatography suppression as of 1993 was summarizedin Rabin, S. et al. J. of Chromatog. 640 (1993) 97-109, incorporatedherein by reference.

SUMMARY OF THE INVENTION

In the present invention, method and apparatus are provided forcontinuously electrolytically suppressing the conductivity of an eluentin an ion exchange bed previously used in separating ions in a separatorbed.

Referring first to the apparatus, the suppressor includes (a) aflow-through suppressor bed of ion exchange resin having exchangeableions of one charge, positive or negative, having a liquid sample inletand an outlet section in fluid communication with suppressor inlet andoutlet ports, respectively, (b) a first electrode chamber disposedadjacent to said suppressor inlet section and having fluid inlet andoutlet ports, (c) a first electrode disposed in said first electrodechamber, (d) a barrier separating said suppressor bed from said firstelectrode chamber, said barrier preventing significant liquid flow butpermitting transport of ions only of the same charge as said suppressorbed resin exchangeable ions, (e) a second electrode in electricalcommunication with said resin bed outlet section, and (f) a recycleconduit providing fluid communication between said suppressor outletport and said electrode chamber inlet port.

Opposite faces of the barrier are in electrical communication with thefirst and second electrodes, respectively, in direct contact or throughconductive medium. For example, the second electrode is in electricalcommunication with the barrier through the conductive suppressor bed.

The suppressor is normally used in combination with (g) a flow-throughseparator bed of ion exchange resin having exchangeable ions of oppositecharge to the exchangeable ions of said suppressor bed, said separatorbed having a sample inlet port and an outlet port, said separator bedoutlet port being in fluid communication with said suppressor bed inletport, and with a detector disposed in the path of said recycle conduitto detect sample flowing through said conduit.

In one embodiment, the second electrode is disposed in contact with saidion exchange resin in said suppressor outlet section. In anotherembodiment, the suppressor combination includes (h) a second electrodechamber disposed adjacent to said suppressor outlet section and havingfluid inlet and outlet ports, and (i) a second barrier separating saidsuppressor bed from said second electrode chamber, said barrierpreventing significant liquid flow but permitting transport of ions onlyof the same charge as said suppressor bed resin exchangeable ions, saidsecond electrode being disposed in said second electrode chamber.

For anion analysis, the suppressor bed ion exchange resin is a cationexchange resin, the first electrode is a cathode, and the secondelectrode is an anode. The opposite polarities apply for cationanalysis.

Referring to one embodiment of the method, anion analysis is performedby the following steps: (a) flowing an aqueous liquid sample streamcontaining anions to be detected and cation hydroxide through aseparator bed of anion exchange resin with exchangeable anions to formliquid effluent including separated sample anions and said cationhydroxide,(b)flowing said aqueous effluent from said separator bedthrough a flow-through suppressor and comprising cation exchange resinincluding exchangeable hydronium ions, so that said cation hydroxide isconverted to weakly ionized form, and some of said exchangeablehydronium ions are displaced by cations from said cation hydroxide, saidsuppressor bed having inlet and outlet sections and inlet and outletports, liquid effluent from said suppressor bed flowing through saidoutlet port,(c) flowing the effluent liquid from said suppressor past adetector in which said separated sample anions are detected, (d)recycling said liquid effluent from said detector through a cathodechamber proximate to said suppressor bed inlet section and separated bya first barrier therefrom, said first barrier substantially preventingliquid flow between said cathode chamber and said suppressor bed inletsection while providing a cation transport bridge therebetween, and (e)applying an electrical potential between a cathode in said cathodechamber and an anode in electrical communication with said suppressorbed outlet section, whereby water is electrolyzed at said anode togenerate hydronium ions to cause cations on said cation exchange resinto electromigrate toward said barrier and to be transported across saidbarrier toward said cathode in said cathode chamber while water in saidcathode chamber is electrolyzed to generate hydroxide ions which combinewith said transported cations to form cation hydroxide in said cathodechamber.

In another embodiment, the liquid effluent is recycled through an anodechamber proximate to said suppressor bed outlet section and separated bya barrier of the same type as the first barrier. The anode is disposedin the anode chamber.

Cation analysis is performed by the same methods with a correspondingreversal of polarity and resin and barriers of opposite charge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of one system according to the presentinvention using a continuous electrolytically regenerated (CER) packedbed suppressor.

FIG. 2 is a schematic view of a two electrode chamber CER packed bedsuppressor according to the invention.

FIGS. 3-5 are chromatograms illustrating use of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In general, the present invention relates to ion chromatography usingcontinuous electrochemical regeneration of a packed bed suppressor.Method and apparatus are provided using electrolytic regeneration of apacked bed suppressor containing ion exchange resin. Ion chromatographyis performed in a conventional manner by chromatographic separation,chemical suppression in a packed bed, and detection. The packed bedsuppressor has electrodes in electrical contact with the resin, whichpermits continuous electrochemical regeneration. The electrodes areseparated from the resin by a barrier which permits ion movement but isimpermeable to liquid flow under typical operating pressures. The devicemay have several ion exchange connectors and electrodes in order toincrease the flux of regenerant ions and eluent counter-ions.Electrochemical regeneration of the packed bed suppressor, byapplication of a direct current (DC) voltage, is continuous during theanalysis by electrolytically splitting an aqueous liquid stream which isseparated from the eluent flow by the ion exchange connectors. Theelectrolytically generated hydronium or hydroxide passes through the ionexchange connector and migrates through the ion exchange resin toneutralize the eluent. Eluent counter-ions pass through the ion exchangeconnector and are swept to waste by the aqueous liquid stream. In oneembodiment, the aqueous liquid stream is the suppressed eluent. Inanother embodiment, the aqueous liquid stream is an independent watersource, preferably deionized water.

The gases created by the electrolysis of the aqueous liquid stream,hydrogen and oxygen, are separated from the eluent flow by the ionexchange connector so that detection is not adversely affected by thegas production.

Apparatus is provided to perform the above continuously regeneratedpacked bed suppressor methods. Such apparatus includes a suppressor withan ion exchange resin bed, liquid barriers that prevent liquid flow butpermit ion transport and means for applying a continuous electricalpotential to electrolyze water in a flowing stream and thus continuouslyregenerate suppressor ion exchange resin to suppress the electrolyte inthe eluent stream.

The system of the present invention is useful for determining a largenumber of ionic species so long as the species to be determined aresolely anions or solely cations. A suitable sample includes surfacewaters, and other liquids such as industrial chemical wastes, bodyfluids, beverages such as fruits and wines and drinking water. When theterm “ionic species” is used herein, it includes species in ionic formand components of molecules which are ionizable under the conditions ofthe present system.

The purpose of the suppressor stage is to reduce the conductivity, andhence noise, of the analysis stream background while enhancing theconductivity of the analytes (i.e., increasing the signal/noise ratio),while maintaining chromatographic efficiency.

In a preferred embodiment, the present invention relates to the use of acontinuous electric field during electrochemical suppression to minimizenoise during detection of the ionic species. Specifically, it has beenfound that the suppressor can be continuously regenerated to convert thechromatography electrolyte to a weakly dissociated form in anuninterrupted manner. When used in this configuration, the requirementfor chemical regenerant is eliminated. Also, the device can toleratehigh system backpressure. Further, it has low noise since theelectrolysis reaction occur in a chamber separate from the eluent flowand reduced manufacturing costs due to the simple design. As usedherein, the term continuous electrolytically regenerated packed bedsuppressor (CERPBS) will refer to this type of system.

In the CERPBS, the electrodes must be in electrical contact with the ionexchange resin either through an ion exchange connector in contact withthe resin or the electrode is directly embedded in the resin. At leastone of the electrodes is separated from the eluent flow path by the ionexchange connector, but still in electrical contact or communicationwith the resin. Also, the barrier is in electrical communication withboth the suppressor bed resin and both electrodes. This configurationpermits eluent counter-ions to be removed from the eluent stream andreplaced with either hydroxide or hydronium to form water or otherweakly conducting aqueous streams. For anion analysis using sodiumhydroxide eluent, the suppressor contains cation exchange resin which iscontinually regenerated to the hydronium ion form by formation ofhydronium ions at the anode, which migrate toward the cathode,displacing sodium ions from the ion exchange sites. At least the cathodeis separated from the eluent stream by the ion exchange connector sothat the sodium ions are removed from the eluent stream and exit thesuppressor as sodium hydroxide. Current is maintained between theelectrodes by movement of ions along ion exchange sites in the ionexchange material in the bed. It is also possible to have the anodeseparated from the eluent by an ion exchange connector. In thisconfiguration, the electrolytically produced hydronium ion passesthrough the ion exchange connector and into the cation resin beingdriven towards the cathode under the force of the electric field. Thisconfiguration permits the continuous regeneration of the suppressorwithout the need to interrupt the analysis cycle to regenerate thesuppressor.

Referring to FIG. 1, an ion chromatography system is illustrated using aCERPBS. The system includes analytical pump 10 connected by tubing 12 tosample injection valve 14 which in turn is connected by tubing 16 to aflow-through chromatographic separator 18 typically in the form of achromatographic column packed with chromatographic resin particles. Theeffluent from chromatographic column 18 flows through tubing 20 to apacked ion exchange resin bed flow-through suppressor 22. Typically,suppressor 22 is formed of a column 24 packed with an ion exchange resinbed 26 of the type used for ion chromatography suppression. Electrodes,in a form to be described below, are spaced apart in the suppressor,with at least one electrode separated from the resin by a barrierdescribed below. The electrodes are connected to a direct current powersupply 27 by leads 27 a and 27 b. The configuration is such that with anaqueous stream flowing through the suppressor and the application ofpower, water in the aqueous stream is electrolyzed to form a source ofhydronium ion or hydroxide ion to continuously regenerate the ionexchange resin bed during the analysis.

The suppressor effluent is directed through tubing 30 to a suitabledetector and then eventually to waste. A preferred detector is aconductivity detector 32 with a flow-through conductivity cell 34. Thechromatography effluent flows through cell 34.

Suppressor 22 generates hydronium ions (and oxygen gas) at the anode andhydroxide ions (and hydrogen gas) at the cathode. If the power supply 26were turned off, the system would operate in the manner of a standardion chromatography system with a packed bed suppressor. That is, awater-containing eluent solution including electrolyte is directed frompump 10 and through tubing 12. Sample is injected through sampleinjection valve 14, and is directed by tubing 16 into chromatographiccolumn 18 to form a first chromatography effluent including separatedionic species of the sample. For simplicity of description, unlessotherwise specified the system will be described with respect to theanalysis of anions using an eluent solution including sodium hydroxideas the electrolyte.

A preferred form of resin is a bed packed with resin particles. However,other forms of resin beds can be used, such as disclosed in thecopending application, incorporated by reference. Suppressor 22 servesto suppress the conductivity of the electrolyte in the eluent suppliedto separator 18 from pump 10 but not the conductivity of the separatedanions. The conductivity of the separated anions is usually enhanced inthe suppression process.

A suitable sample is supplied through sample injection valve 14 which iscarried in a solution of eluent supplied from pump 10. Anode 36 isdisposed at the outlet end of resin bed 26 in intimate contact with theresin therein. The effluent from bed 26 exits through port 37 and isdirected to a detector suitably in the form of a flow-throughconductivity cell 34, for detecting the resolved anions in the effluent,connected to a conductivity meter 32.

In the detector, the presence of anions produces an electrical signalproportional to the amount of ionic material. Such signal is typicallydirected from the cell 34 to a conductivity meter 32, thus permittingthe detection of separated ionic species of interest (anions for anionanalysis).

In a preferred embodiment, detection is by electrical conductivity andso the present system is described using ion conductivity detector.However, other forms of detectors may be used including absorbance, massspectrometry, and inductive coupled plasma. Detection of the presentinvention will be described with respect to a conductivity detector.

The system also includes means for pressurizing the effluent fromsuppressor 22 prior to detection to minimize adverse effect of gases(hydrogen or oxygen) generated in the system as will be describedhereinafter. As illustrated in FIG. 1, such pressurizing means comprisesa flow restrictor 38 downstream of conductivity cell 34 to maintain theion chromatography system under pressure.

Column 24 is typically formed of plastic conventionally used for an ionexchange column. It has a cylindrical cavity of a suitable length, e.g.,60 mm long and 4 mm in diameter. It is packed with a high capacitycation exchange resin, e.g., of the sulfonated polystyrene type. Theresin is suitably contained in the column by a porous frit which servesto provide an outlet to the column. In the illustrated embodiment, theporous frit is porous electrode 36 which serves the dual function ofcontainment of the resin and as an electrode.

Forms of ion exchange beds other than packed resin beds can be used incolumn 24, such as a porous continuous structure with sufficientporosity to permit flow of an aqueous stream at a sufficient rate foruse in chromatography without undue pressure drop and with sufficiention exchange capacity to form a conducting bridge of cations or anionsbetween the electrodes. One form of structure is a porous matrix or asponge-like material formed of sulfonated, cross-linked polystyrene witha porosity of about 10 to 15% permitting a flow rate of about 0.1 to 3ml/min. without excessive pressure drop.

A barrier 40 separates bed 26 from electrode 42 in the interior of ahollow housing defining electrode chamber 44 preventing any significantliquid flow but permitting transport of ions only of the same charge asthe charge of exchangeable ions on resin bed 26. For anion analysis,barrier 40 is suitably in the form of a cation exchange membrane or plugseparating electrode chamber 44 from the cation exchange resin.

Electrode 42 in electrode chamber 44 also suitably is in the form of aninert metal (e.g., platinum) porous electrode in intimate contact withbarrier 40. An electrode is fabricated in a way to permit goodirrigation of the electrode/membrane interface when water is passedthrough electrode chamber 44. The electrode is suitably prepared bycrumpling and forming a length of fine platinum wire so as to produce aroughly disc-shaped object that allows easy liquid flow throughout itsstructure and at the electrode membrane interface. Good contact betweenthe disc-electrode 42 and barrier 40 is maintained simply by arrangingthat the one press against the other. The electrode can extend acrossall or part of the aqueous liquid flow path through electrode chamber 42to provided intimate contact with the flowing aqueous stream.

A conduit 48 is provided to direct the aqueous liquid stream to theinlet 50 of electrode chamber 44. Conduit 52 takes the effluent fromchamber 44 to waste. All conduits may be made from narrow bore plastictubing. However, if desired, conduit 50, 52 and 54 may be made out ofstainless steel tubing. When these metal conduits are allowed to touchthe platinum electrodes, they make electrical contact with theelectrodes as well as being conduits for fluid flow. This provides ameans of making electrical contact with the electrodes that is at thesame time easy to seal against liquid leakage.

The line X—X is illustrated across the resin bed 26. For reasons whichwill be explained below, the resin above the dotted line ispredominantly or substantially completely in the form of the cationcounter ion of the base used as the electrolyte during separation. Belowthe line X—X, the resin is predominantly or completely in the hydroniumform. The line X—X represents the interface. As used herein, the terms“anion or cation or ion exchange beds” refer to flow-through beds ofanion or cation exchange material through which the aqueous liquidstream flows. Unless otherwise stated, the term “cation” excludeshydronium ions and the term “anion” excludes hydroxide ions. Because ofits ready availability and known characteristics, a preferred form ofion exchange bed is a packed ion exchange bed of resin particles. It isdesirable that the resin particles be tightly packed in the bed, to forma continuous ion bridge or pathway for the flow of ions betweenelectrodes 36 and 42. Also, there must be sufficient spacing for theaqueous stream to flow through the bed without undue pressure drops.

As defined herein, the portion of bed 26 above the line X—X is referredto as the suppressor bed inlet section 26 a. Conversely, the portion ofthe bed below the line X—X is referred to as the suppressor bed outletsection 26 b. As illustrated, barrier 40 of electrode chamber 44 isdisposed adjacent bed inlet section 26 a and, therefore, primarily is inthe cation form.

The principle of operation of the system for anion analysis is asfollows. An aqueous liquid stream containing anions to be detected and acation (e.g., potassium) hydroxide flows through separator bed 18 ofanion exchange resin with exchangeable anions to form a liquid effluentincluding separated anions and the cation hydroxide. Anion exchangeresin in bed 18 is of a suitable conventional low capacity form used forion chromatography as illustrated in U.S. Pat. Nos. 3,897,213,3,920,397, 3,925,019 and 3,926,559. For example, bed 18 has typically atotal capacity of about 0.01 to 0.1 milliequivalents. As isconventional, the anion exchange capacity of the separator is low incomparison to that of the suppressor.

The ratio of the capacities of the ion exchange resin in suppressor bed26 to separator bed 18 may be the same as used for ion chromatographyusing a conventional packed bed suppressor, e.g. from 10:1 to 1000:1.

For anion analysis, a polarizing DC potential is applied between cathode42 and anode 36, and the following reactions take place.

The water is electrolyzed and hydronium ions are generated at anode 36according to the following reaction:

H₂O−2e→2H⁺+½O₂→.  (1)

This causes cations in the cation exchange resin bed 26 to migrate tobarrier 40. This, in turn, displaces hydronium ions upwardly through bed26 which causes a similar displacement of cations ahead of them. Thecations electromigrate toward the barrier 40 to be transported acrossthe barrier 40 toward cathode 42 in cathode chamber 44 while water iselectrolyzed at cathode 42 to generate hydroxide ions according to thefollowing reaction:

2 H₂O+2e→2OH⁻+H₂→.  (2)

The cations which have transported across the barrier combine with thegenerated hydroxide ions to form cation hydroxide in cathode chamber 44.The effluent from separator bed 60 exits through outlet port 37 andconduit 30 and percolates through the cation form resin in inlet bedsection 26 until it reaches the hydronium form resin in bed section 26where it is neutralized while the cation is retained on the resin. Atthis point, the anion salts are converted to their respective acids andthe cation hydroxide is converted to weakly ionized form, water.

The suppressed effluent liquid containing the separated anions leavesbed 26 through port 27 and conduit 30 and passes to conductivity cell 34in which the conductivity of the separated anions is detected.

The effluent from conductivity cell 34 passes through flow restrictor 38and conduit 48 and is recycled to electrode chamber 44. This provides asource of aqueous liquid to permit continuous reaction in electrodechamber 44 by passing the formed acid or base to waste in a continuousstream.

The net result of the electrode reactions and the electromigration ofthe resin counterions are: the production of cation (e.g., potassium)hydroxide in the region of the cathode, and electrolytic gases at thetwo electrodes. Specifically, the electrode reactions produce, hydrogenand oxygen which are carried out of the suppressor into thechromatography system.

When the hydronium ion/cation boundary line X-X is reached, the cation(shown as potassium) hydroxide is neutralized as a conventionalsuppression according to the following equation:

KOH+H⁺R⁻→K⁺R⁻+H₂O,  (3)

wherein R is the cation exchange resin. The K⁺R⁻ indicates that the ionexchange resin retains the cation as its exchangeable ion.

The flux of hydronium “upwards” in the resin phase toward bed inletsection 26 a is equivalent to or greater than the flux of cationhydroxide “downwards” in the mobile phase toward bed outlet section 26b. Since the balance prevails at different current levels, the positionof the hydronium/cation boundary line X—X remains fixed. Thus, thesystem operates as a continuous suppressor of cation hydroxide.

The system of FIG. 1 has been described with respect to a system for theanalysis of anions. However, the system is also applicable to theanalysis of cations. In this instance, electrode 36 is a cathode andelectrode 42 is an anode. The polarity type resin is reversed. Thus, theresin in separator bed 18 is a cation exchange resin and the resin insuppressor bed 26 is an anion exchange resin. The plug or membrane 40 isan ion exchanging material.

Briefly described, the system works as follows for the cation analysis.The aqueous liquid stream containing cations to be detected and an acidelectrolyte aqueous eluent are directed through separator bed 18including cation exchange resin. The effluent from separator bed 18flows through suppressor bed 26 including anion exchange resin withexchangeable hydroxide ions. The acid in the eluent is converted toweakly ionized form. Some of the exchangeable hydroxide is displaced byanions from the acid.

An electrical potential is applied between the cathode 36 and anode 42.Water is electrolyzed at electrode 36 to generate hydroxide to causeanions on the anion exchange resin bed to electromigrate toward barrier40 to be transported across the barrier toward the positively chargedanode 42 in the electrode chamber 44 while water in chamber 44 iselectrolyzed to generate hydronium ions which combine with thetransported anions to form acid in the electrode chamber 44. Theeffluent liquid from the suppressor bed 26 flows past detector 32 inwhich separated cations are detected and is recycled to electrodechamber 44.

The exchangeable cations or anions for suppressor bed 26 and, thus forthe acid or base electrolyte in the aqueous eluent, must also besufficiently water soluble in base or acid form to be used at thedesired concentrations. Suitable cations are metals, preferably alkalimetals such as sodium, potassium, lithium and cesium. Known packing forhigh capacity ion exchange resin beds are suitable for this purpose.Typically, the resin support particles may be in the potassium or sodiumform. Potassium is a particularly effective exchangeable cation becauseof its high conductance. Suitable other cations are tetramethyl ammoniumand tetraethyl ammonium. Analogously, suitable exchangeable anions forcation analysis include chloride, sulfate and methane sulfonate.Typically, resin support particles for these exchangeable anions includeDowex 1 and Dowex 2.

Another embodiment of the invention is illustrated in FIG. 2. Like theembodiment in FIG. 1, the FIG. 2 embodiment may be used with aconventional packed ion exchange resin bed separator column. Theprincipal difference between the embodiments of FIGS. 1 and 2 is that inthe latter one, there are two external electrode chambers rather thanone so that the analyte ions are prevented from contacting anyelectrodes.

FIG. 2 schematically illustrates only that portion of the systemdownstream from the separation column. In the illustrated system theeffluent from the separator column flows in conduit 60 throughsuppressor 62 which includes a housing (suitably of cylindricalcross-section) including a body 64 defining a central bore, screwthreaded top and bottom caps 66 and 68, and top and bottom flow-throughbed supports 70 and 72, respectively, at opposite ends of the bore.Suppressor 62 contains a high-capacity ion exchange resin bed 68 of thetype described above. Electrode chamber 70 contains electrode 72separated from bed 68 by barrier 74, all of the same type describedabove. The difference from FIG. 1 is that FIG. 2 includes an electrodechamber 76 containing a second electrode 78 separated by barrier 80 frombed 68. Both electrode chambers 70 and 76 may be of the same type withthe exception that the electrodes are of opposite polarity. Electrode 78in electrode chamber 76 replaces electrode 36 in FIG. 1 which was indirect contact with the resin bed of the suppressor. The electrodes areconnected to a DC power supply, not shown, and are suitably formed ofplatinum. Beds supports 70 and 72 are first positioned and then caps 66and 68 are screwed into a secure position. As is conventional, the endcaps include screw-threaded ports for connecting to the inlet and outlettubing.

The effluent from suppressor 62 flows through line 82 and throughdetector 84 and line 86 to electrode chamber 76. The effluent fromelectrode chamber 76 is further recycled through line 88 to the inletside of electrode chamber 70. The effluent from electrode chamber 70passes through line 90 to waste.

In this embodiment, resin bed 68, electrodes 72 and 78, and barriers 74and 80 are in electrical communication. However, barriers 74 and 80separate the sample eluent flow through the suppressor 62 from theliquid flow in the anode and cathode chambers. The same reactions as inFIG. 1 occur at the anode and cathode. Specifically, for anion analysis,the foregoing description applies to the reaction in cathode chamber 70.It is disposed above the line X—X in the inlet portion 68 a of bed 68.Similarly, the same reaction occurs at anode 78 as described in FIG. 1with respect to anode 36. However, the presence of the barrier createsthe following difference in operation. The hydronium ions generated atanode 78 electrophoretically pass through barrier 80 to the cationexchange resin in bed 68 where they are driven upwardly in the resintoward cathode 72 in the manner described above. Similarly, cations fromthe eluent are displaced from the cation exchange resin by the upwardflux of hydronium ions which combine with the eluent hydroxide to formwater. This reaction scheme is as also set forth above. Similarly, thecations are electrophoretically driven through barrier 74 to electrode72 where they associate with the hydroxide ions to form a base forpassage to waste. The oxygen produced at the anode compartment and thehydroxide produced in the cathode department are swept away with thesodium hydroxide. Thus, no flow restrictor is necessary to minimize theeffect of such gases on analysis since the gases are separated from theanalytical system.

Another advantage of separating the anode and cathode by barriers 74 and80 is that the eluent stream flowing through bed 68 does not pass overthe electroactive surface of the electrodes where a solvent or analytecould be electrochemically modified. This can be important when anorganic modifier is used in the eluent. For example, methanol, a commonorganic modifier with sodium hydroxide eluents, can be oxidized at theanode to formic acid which raises the background conductivity. With theelectrode separated from the eluent compartment by the barriers, theeluent stream is not exposed to undesirable electrochemical reactions.

The proper operating current for the CERPBS depends on the eluentcomposition. For the anion example, the electrochemically generatedhydronium flux must be greater than or equal to the incoming sodiumhydroxide flux. This assures that every mole of hydroxide is neutralizedby a mole of hydronium and that the sodium is displaced by hydroniumthrough the ion exchange connector, to the cathode compartment which isswept to waste. Typically, the current is 110-160% of the eluent flux.

The operating voltage depends on the device geometry, electrode size,electrode spacing as well as the resin and ion exchange connectorconductivity.

The device is designed to minimize the voltage drop and typicaloperating voltages range from 10 to 100 volts. It is generally desirableto operate the device in the constant current mode since current can bedirectly related to the eluent concentration, and hence the regenerantflux required.

An important feature of the suppressor is the use of means for applyingan electrical potential through the ion exchange connectors and acrossthe ion exchange resin. Any number of configurations may be employed solong as the potential is applied to a significant part of the resin forefficient regeneration and the eluent cations are removed through theion exchange connector. In that regard, the anode and cathode should bespaced apart with the majority of the ion exchange resin disposedtherebetween.

The following examples illustrate different aspects of the presentinvention.

EXAMPLE 1

This example illustrates the use of an continuous electrolyticallyregenerated packed bed suppressor of the type illustrated in FIG. 1.This example is given for the suppression of sodium hydroxide which isused as an eluent for anion separations. As shown in FIG. 1, aconventional chromatographic system (Dionex Corp., Sunnyvale, Calif.)was used consisting of a pump 10, with injection valve 14 connected toan ion exchange separator column 18. In this experiment, a Dionex anionseparator, IonPac AS11 was used. A continuous electrolyticallyregenerated packed bed suppressor 22, as described in this disclosure,was used. Suppressor 22 includes central flow channel that is 4×70 mmcolumn. Suppressor 22 was packed with 20 μ fully sulfonatedpolystyrene/8% divinylbenzene which was packed in the sodium form andthen converted to the hydronium form with sulfuric acid. A cationexchange membrane AMI-7000 (217 in FIG. 2) from Membrane International,NJ was used as barrier 40 in electrode chamber 44. A power supply fromHoefer Scientific Instruments (CA), Model PS2500, 27, was used to applya DC voltage to platinum electrodes, 36 and 42. Anode 36, in porousplatinum form, was placed at the outlet of suppressor 22 which also actsas a flow-through bed support to retain the resin. A conductivitydetector and cell, 32, 34 was used to monitor the effluent from thesuppressor. Backpressure was applied to the cell using 15 cm of 0.076 mmid PEEK tubing as restrictor 170. Data was collected using Dionex AI450Chromatography software.

EXAMPLE 2

In order to demonstrate the dynamic suppression capacity of the devicedescribed in Example 1, a gradient separation of anions was performed.In this example, the maximum eluent concentration is 30 mM NaOH. Thechromatogram shown in FIG. 4 was obtained under the followingconditions:

Column: IonPac AS11

Flow rate: 1.0 mL/min

Eluent: Gradient from 1 mM NaOH to 30 mM NaOH over 15 minutes

Injection: 25 μL of 2 ppm F⁻, 3 ppm Cl⁻ and 15 ppm NO₃ ⁻ and 15 ppm SO₄²⁻

Applied voltage: 45V

Current: 65 mA

EXAMPLE 3

This example illustrates the use of an continuous electrolyticallyregenerated packed bed suppressor of the type illustrated in FIG. 2.This example is given for the suppression of sulfuric acid which is usedas an eluent for cation separations. As shown in FIG. 1, a conventionalchromatographic system (Dionex Corp., Sunnyvale, Calif.) was usedconsisting of a gradient pump 10, with injection valve 14 connected toan ion exchange separator column 18. In this experiment, a Dionex cationseparator, IonPac CS12A was used. A 25 continuous electrolyticallyregenerated packed bed suppressor, 62 as shown in FIG. 2 and describedin this disclosure was used. The suppressor includes central flowchannel that is 4×70 mm column. The suppressor was packed with 20 μfully aminated vinylbenzylchloride-8%divinylbenzene resin which waspacked in the hydroxide form. An anion exchange membrane AMI-7001 (74and 80 in FIG. 2) from Membrane International, NJ was used in theelectrode chambers, 70 and 76. A power supply from Hoefer ScientificInstruments (CA), Model PS2500, 27, was used to apply a DC voltage toplatinum electrodes, 72 and 78. A conductivity detector and cell, 84 wasused to monitor the effluent from the suppressor. Data was collectedusing Dionex AI450 Chromatography software. Using the above apparatusand the conditions listed below, the chromatogram in FIG. 5 wasobtained. The background conductivity of the suppressed eluent was about0.4 μS-cm indicating complete suppression of the 18 mN sulfuric acideluent.

Column: IonPac CS12A

Flow rate: 1.0 mL/min

Eluent: 18 mN H₂SO₄

Injection volume: 25 μL of 0.5 ppm Li⁺, 2 ppm Na⁺, 2.5 ppm NH₄ ⁺, 5ppmK⁺, 2.5 ppm Mg²⁺, 5.0 ppm Ca²⁺

Applied voltage: 47V

Current: 60 mA

A chromatogram of the results is illustrated in FIG. 5.

EXAMPLE 4

In this example, a flow-through sponge-like cation exchange bed isformed to act as a suppressor for anion analysis.

Styrene and divinylbenzene are copolymerized in the presence of anappropriate catalyst and a porogen. A porogen is an added materialwhich, when removed after the polymerization is complete, creates amacroporosity in the polymerized structure. This porosity should be suchthat it provides for a ready flow of liquids through the polymer phasewhile at the same time providing adequate areas of contact between thepolymer and liquid phase. The porogen can be a finely divided solidwhich can be easily removed by dissolution in acid or base (e.g.,calcium carbonate or silica), or it can be a solvent which is rejectedby the polymer as it forms and is subsequently displaced by anothersolvent or water. Suitable liquid porogens include an alcohol, e.g.,used in the manner described in Analytical Chemistry, Vol. 68, No. 2,pp. 315-321, Jan. 15, 1996.

After the porogen is removed, the polymer is sulfonated by commonlyknown sulfonating agents such as concentrated sulfuric acid orchlorosulfanic acid.

A suitable shape for the polymer is a cylindrical rod which, aftersulfonation and conversion to the suitable metal ion form can be placedin the cylindrical cavity of the suppressor column. Preferably, the ionexchange rod is introduced into the column in a slightly shrunken formso that in its typical use environment it swells to form a tight fitwith the wall of the column and the cation exchange membrane(s) thatseparate the ion exchange rod from the electrode compartment(s).

As a final step, the rod is treated so that the part closest to theoutlet is in the hydronium form while the part closest to the inlet isin a metal cation form such as the potassium form. This is accomplishedby treating the rod with the appropriate amount of acid, or byelectrochemically displacing potassium ions with hydronium ions.

What is claimed is:
 1. A suppressor for ion chromatography comprising:(a) a flow-through suppressor bed of ion exchange resin havingexchangeable ions of one charge, positive or negative, having suppressorbed inlet and outlet sections in fluid communication with suppressorinlet and outlet ports, respectively, (b) a first electrode chamberdisposed adjacent to said suppressor bed and having fluid inlet andoutlet ports, (c) a first electrode disposed in said first electrodechamber adjacent to said suppressor bed inlet section but not adjacentto said suppressor bed outlet section, (d) a first barrier separatingsaid suppressor bed from said first electrode chamber, said barrierpreventing significant liquid flow but permitting transport of ions onlyof the same charge as said suppressor bed resin exchangeable ions, saidfirst electrode being in electrical communication with said firstbarrier, (e) a second electrode disposed in or adjacent to saidsuppressor bed outlet section in electrical communication therewith butnot in or adjacent to said suppressor bed inlet section, and (f) arecycle conduit providing fluid communication between said suppressoroutlet port and said first electrode chamber inlet port.
 2. Thesuppressor of claim 1 in combination with: (g) a flow-through separatorbed of ion exchange resin having exchangeable ions of opposite charge tothe exchangeable ions of said suppressor bed, said separator bed havinga sample inlet port and an outlet port, said separator bed outlet portbeing in fluid communication with said suppressor bed inlet port.
 3. Thesuppressor combination of claim 2 further comprising a detector disposedin the path of said recycle conduit to detect sample flowing throughsaid conduit.
 4. The suppressor combination of claim 2 in which saidsecond electrode is disposed in contact with said ion exchange resin insaid suppressor bed outlet section.
 5. The suppressor combination ofclaim 2 further comprising: (h) a second electrode chamber disposedadjacent to said suppressor bed and having fluid inlet and outlet ports,and (i) a second barrier separating said suppressor bed from said secondelectrode chamber, said barrier preventing significant liquid flow butpermitting transport of ions only of the same charge as said suppressorbed resin exchangeable ions, said second electrode being disposed insaid second electrode chamber adjacent to said suppressor bed outletsection.
 6. The suppressor combination of claim 5 in which said secondbarrier is adjacent to said suppressor bed outlet section but notadjacent to said suppressor bed inlet section.
 7. The suppressorcombination of claim 5 in which said recycle conduit also provides fluidcommunication with said second electrode chamber inlet port so thatfluid in said recycle conduit flows sequentially from said suppressoroutlet port through said second electrode chamber and then to said firstelectrode chamber.
 8. The suppressor of claim 1 in which said suppressorbed ion exchange resin is a cation exchange resin, said first electrodeis a cathode, and said second electrode is an anode.
 9. The suppressorof claim 1 in which said suppressor bed ion exchange resin is an anionexchange resin, said first electrode is an anode, and said secondelectrode is a cathode.
 10. The suppressor of claim 1 in which saidfirst barrier is adjacent to said suppressor bed inlet section but notadjacent to said suppressor bed outlet section.